add High Sierra Mac OSX support

This is an issue covering both build and execution issues on High Sierra.

The cmake through brew is now 3.12.3 which hits a new policy warning I added to #1375.

/Users/derek/drmemory/git/src/drsyscall/table_macos_bsd.c:43:10: fatal error: 'security/mac.h' file not found
#include <security/mac.h>
         ^~~~~~~~~~~~~~~~

/Users/derek/drmemory/git/src/tests/syscalls_unix.c:172:5: warning: 'syscall' is deprecated: first deprecated in
      macOS 10.12 - syscall(2) is unsupported; please switch to a supported interface. For SYS_kdebug_trace use
      kdebug_signpost(). [-Wdeprecated-declarations]
    syscall(SYS_finit_module, fd, stack_str.str, 0);
    ^
/usr/include/unistd.h:745:6: note: 'syscall' has been explicitly marked deprecated here
int      syscall(int, ...);
         ^
/Users/derek/drmemory/git/src/tests/syscalls_unix.c:184:5: warning: 'syscall' is deprecated: first deprecated in
      macOS 10.12 - syscall(2) is unsupported; please switch to a supported interface. For SYS_kdebug_trace use
      kdebug_signpost(). [-Wdeprecated-declarations]
    syscall(SYS_finit_module, -1, dangling_ptr->str, 0);
    ^
/usr/include/unistd.h:745:6: note: 'syscall' has been explicitly marked deprecated here
int      syscall(int, ...);
         ^
/Users/derek/drmemory/git/src/tests/syscalls_unix.c:236:13: warning: 'syscall' is deprecated: first deprecated in
      macOS 10.12 - syscall(2) is unsupported; please switch to a supported interface. For SYS_kdebug_trace use
      kdebug_signpost(). [-Wdeprecated-declarations]
        n = syscall(SYS_process_vm_readv, getpid(), local, 2, remote, 1, 0);
            ^
/usr/include/unistd.h:745:6: note: 'syscall' has been explicitly marked deprecated here
int      syscall(int, ...);
         ^
/Users/derek/drmemory/git/src/tests/syscalls_unix.c:238:13: warning: 'syscall' is deprecated: first deprecated in
      macOS 10.12 - syscall(2) is unsupported; please switch to a supported interface. For SYS_kdebug_trace use
      kdebug_signpost(). [-Wdeprecated-declarations]
        n = syscall(SYS_process_vm_writev, getpid(), local, 2, remote, 1, 0);
            ^
/usr/include/unistd.h:745:6: note: 'syscall' has been explicitly marked deprecated here
int      syscall(int, ...);

<Application /Users/derek/DrMemory-MacOS-1.11.0-2/executable_path=/Users/derek/dr/test/hello32 (7684) is running on an unsupported operating system (17.7.0)>

After Stopping message when running 1.11.0-2/dynamorio/bin32/drrun on hello,world it hangs.

lldb shows it's hung on read_lock(&callback_registration_lock) b/c the lock_requests is a bogus value:

(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = signal SIGSTOP
  * frame #0: 0xa73e14fe libsystem_kernel.dylib`swtch_pri + 10
    frame #1: 0x7156edd7 libdynamorio.dylib`os_thread_yield at os.c:2981
    frame #2: 0x715871e1 libdynamorio.dylib`rwlock_wait_contended_reader(rwlock=0x71676c1c) at os.c:8864
    frame #3: 0x71253110 libdynamorio.dylib`read_lock(rw=0x71676c1c) at utils.c:1155
    frame #4: 0x7140ccd5 libdynamorio.dylib`instrument_exit at instrument.c:765
    frame #5: 0x71096eb8 libdynamorio.dylib`dynamo_shared_exit at dynamo.c:974
    frame #6: 0x71097ec9 libdynamorio.dylib`dynamo_process_exit_cleanup at dynamo.c:1241
    frame #7: 0x7109785e libdynamorio.dylib`dynamo_process_exit at dynamo.c:1297
    frame #8: 0x71542bc4 libdynamorio.dylib`cat_done_saving_dstack + 25
(lldb) frame select 3
frame #3: 0x71253110 libdynamorio.dylib`read_lock(rw=0x71676c1c) at utils.c:1155
(lldb) fr v -f x *rw
(read_write_lock_t) *rw = {
  lock = {
    lock_requests = 0x205c85f5
    contended_event = (sem = 0x00000a28, value = 0x00000000)
    name = 0x71625b76 "callback_registration_lock(readwrite)@/drmemory_package/dynamorio/core/lib/instrument.c:120"
    rank = 0x0000002b
    owner = 0x0000000000000000
    owning_dcontext = 0x00000000
    prev_owned_lock = 0x00000000
    count_times_acquired = 0x00000002
    count_times_contended = 0x1b3cb505
    count_times_spin_pause = 0x00000000
    max_contended_requests = 0x00000000
    count_times_spin_only = 0x00000000
    prev_process_lock = 0x71676158
    next_process_lock = 0x4c51f58c
    callstack = ([0] = 0x00000000, [1] = 0x00000000, [2] = 0x00000000, [3] = 0x00000000)
    app_lock = 0x00
    deleted = 0x00
  }
  num_readers = 0x00000000
  writer = 0x0000000000000000
  num_pending_readers = 0x00000001
  writer_waiting_readers = (sem = 0x00000000, value = 0x00000000)
  readers_waiting_writer = (sem = 0x00000000, value = 0x00000000)
}

Error #1: UNADDRESSABLE ACCESS of freed memory: reading 0x0041f0f3-0x0041f0f4 1 byte(s)
# 0 main               [/Users/derek/drmemory/git/src/tests/free.c:81]
Note: <FB>0:00:00.000 in thread 100688
Note: 0x0041f0f3-0x0041f0f4 overlaps memory 0x0041f0f0-0x0041f104 that was freed here:
Note: # 0 replace_free               
Note: # 1 main                       [/Users/derek/drmemory/git/src/tests/free.c:77]
Note: instruction: mov    0x03(%eax) -> %cl